Guide cameras may be divided into two categories. First contains cheap Micron CMOS based cameras. Second one contain Sony CCD based 16-bit cameras. Sensitivity of such cameras may be comparable but sensor noise, diagonal or electronics may have great impact on the ability to provide good guide star images. In this article I'm going to compare QHY5 and Atik Titan - I'm going to check the image they provide in PHD.
QHY5 is very popular and quite cheap guide camera. The Micron/Aptina CMOS sensor has a 1/2" diagonal. Atik Titan costs much more than QHY5 and it is equipped with a CCD having 1/3" diagonal and slightly bigger pixels.
Bigger diagonal will cover bigger part of the sky, while bigger pixel in Atik will give better signal response as more photons will hit the same pixel. The resolution will be slightly lower but in most guide scopes or OAGs it won't be a problem at all.
The main difference between those two cameras is electronics. Titan uses 16-bit A/D and low gain. QHY5 uses high gain and 10-bit (working as a 8-bit usually) A/D converter. Micron CMOS has a much higher read noise than ICX424 CCD so the noise in QHY5 will be noticeably higher. Will it affect star finding?
QHY5 provided bigger sky coverage so more stars could fit in. When increasing exposure time Atik provided better and better frame quality, while QHY5 improved the image only partially. Even so both cameras succeeded in providing gude stars. If you often have very faint guide stars and you can use few sec exposures for the guider - Atik Titan will be better. If you can find guide stars easily then QHY5 will perform equally good.